The higher-order harmonic filters are needed for creation of class-F RF power amplifiers. These filters are required both to increase the efficiency of the amplifiers by reflecting the power carried by odd harmonics and to combine the power from several amplifiers (RF modules) and deliver it the load. The demand for such device is especially high for designing compact high-efficient RF generators/amplifiers.
Currently, there exist several patents concerning this problem: U.S. Pat. No. 4,562,409, U.S. Pat. No. 4,926,145, U.S. Pat. No. 6,242,984B1, U.S. Pat. No. 4,238,747 etc. All mentioned patents describe devices that solve the problem of parallel power combining with suppression of unwanted higher-order modes, propagating at frequencies higher than the operating one. The radial transmission line, to which the plurality of RF modules is connected, and output coaxial line is used in all patents.
U.S. Pat. No. 4,238,747 discloses a mode filter apparatus for preventing unwanted modes in a multi-moded structure utilizing a radial plurality of resonant slots on the bottom of the filter cavity. The top of the filter cavity includes a variable height center section to provide mode selectivity in cooperation with the resonant slots. The cylindrical resonant cavity has a radial pattern of resonant slots in the bottom plate and a variable height center portion in the top plate of the apparatus. The mode selectivity of the mode filter apparatus may be controlled by varying the height of the center portion of the top plate, as well as by changing the length, width and depth of the radial slots. The mode selectivity may also be varied by filling the radial slots with absorbing material to various depths.
U.S. Pat. No. 4,926,145 discloses a radial power combiner/divider in which energy corresponding to undesired higher order modes in the radial transmission line, including the m=1 and m=2 modes, is effectively absorbed. In said radial power combiner/divider radial slots are provided for suppression of undesired modes, certain undesired modes which are not adequately suppressed by the radial slots are allowed to be propagated in a central coaxial transmission line and suppressed therein by means of longitudinal slots in the outer conductor. In an alternative embodiment, the central transmission line of the combiner/divider is in the form of a circular waveguide, and the suppression means comprises thin, spaced coaxial cylinders of dissipative material.
U.S. Pat. No. 4,562,409 discloses a cavity resonator coupling-type power distributor/power combiner which can distribute or combine microwave electric power over a wide bandwidth with a small insertion loss. A cavity resonator coupling-type power distributor/power combiner includes a first cavity resonator operatively resonating with a cylindrical TO,n,O mode, and a plurality of second cavity resonators arranged on the periphery of the first cavity resonator and extending radially and symmetrically with respect to the first cavity resonator. The second cavity resonators each have the same shape and size so that magnetic-field coupling is established between the first cavity resonator and each of the second cavity resonators, for distributing or combining microwave power in a microwave amplifier.
In the cavity resonator the higher harmonics are suppressed with radial slits in conductors of the radial line and longitudinal slits in the outer conductor of the output coaxial line; all slits are filled with absorbing dissipative material. These patents solve the problem of power combining and signal filtration. In mentioned patents, the power of the higher harmonics either dissipates in combiner walls or in dielectric material, but is not reflected back to the RF modules as it is claimed in present embodiments of the invention disclosure.
U.S. Pat. No. 6,242,984 relates to solid state power amplifier (SSPA) modules that splits a signal into multiple parts, uses distributed amplifiers to amplify the parts, and recombines the amplified parts into a single output.
An SSPA module in accordance with the present embodiment of the invention comprises a signal input, and a radial splitter connected to the signal input comprising a plurality of radially extending splitter waveguides. The SSPA module also includes a signal output, and a radial combiner connected to the signal output comprising a plurality of radially extending combiner waveguides. Connections between the splitter and combiner are provided by a plurality of vertically extending waveguides. The SSPA module also includes a plurality of processing circuits for example MMIC amplifiers, connected to the combiner waveguides. A waveguide to microstrip transition may also be used to connect signals propagating in the waveguides to and from microstrip lines connected to the processing circuitry. Generally, the transition includes a waveguide section with a top conducting layer that defines a first slit and a second slit bounding a transition area abutting a microstrip section to form a waveguide to microstrip transition.
In patent U.S. Pat. No. 4,562,409 as in U.S. Pat. No. 6,242,984B1 the signal from each RF module is first injected to a cavity resonator based on rectangular waveguide, which is coupled via magnetic field to a radial line segment with coaxial output. That design implies two resonators in series, which is possible to use for the reflection of the third harmonic.